A reflectometer is an optical system used to measure diffusely reflected light. Reflectometers are generally used in applications of diffuse reflectance spectroscopy. These applications cover the gamut from routine quality control testing to basic research. Reflection densitometers, for example, measure the negative logarithm of the diffuse reflectance of a sample in one of several specified sprectral bands. They are used in the measurement of such materials as photographic prints, printing ink on paper, plastic or metal substrates, and thin layer chromatograms. Reflection spectrophotometers generally measure diffuse reflectance in many narrow spectral bands for such applications as the measurement of the color of paints, plastics and textiles, and the analysis of powdered chemicals or biological specimens. The present invention relates to the illumination and collection optics for a diffuse reflectometer.
Diffuse reflectance, insofar as the preferred embodiment of the present invention is concerned, is defined as the ratio of light flux reflected normal to a sample to the light flux diffusely incident upon it. Diffuse reflectance can also be defined in terms of normal illumination and diffuse collection.
Thus, a diffuse reflectance measurement excludes specular reflections. Diffuse illumination is usually provided in one of two forms--sphere illumination or 45.degree. illumination. In sphere illumination the illumination comes from an integrating sphere and is incident upon the sample from all directions (in a hemisphere). In 45.degree. illumination the incident light is concentrated in an annulus that impinges at 45.degree. to the sample, with a slight angular spread about 45.degree.. The diffusely reflected light is collected normal to the sample, with a slight angular spread about the normal. This illumination/collection geometry is commonly called 45.degree./0.degree. and is the subject of the preferred embodiment of the present invention.
The illumination optics provide illumination from a complete annular ring, the rays of such illumination having an angle of incidence lying in a prescribed angular spread about 45.degree. to the sample normal. Furthermore, the illumination incident upon the sample is of uniform irradiance over a prescribed circular region of the sample. The collection optics receive light that is diffusely reflected from the sample in a cone of prescribed half-angle about the sample normal. It furthermore restricts such collection to the same circular region of the sample that has been uniformly illuminated. The collection optics also provide for spectral filtering of the reflected light and can direct this light via an optical fiber to a photodetector in a remote location.
The entire optical system is comprised of elements that are relatively simple to manufacture or readily available at low cost and does not involve the use of any high quality optical elements. There is no restriction on the size of the optical system so that it can be used for making diffuse reflectance measurements on small or large area samples, whether stationary or moving, and need not contact the sample. In addition, the optical system meets the geometrical conditions specified by the American National Standards Institute for diffuse reflectance measurements on photographic and graphic arts materials.